1. Field of the Invention
This invention relates to systems for measuring the load carried by vehicles and, more particularly, to a measuring system having strain sensing devices mounted on vehicle suspension members for measuring deflection of the members responsive to loading of the vehicle.
2. Description of the Prior Art
Weight regulations limiting the loaded weights of commercial vehicles are almost universally in effect. These regulations generally specify the maximum load of each vehicle axle as well as the maximum total load. Fines are usually levied against operators who are found violating these regulations.
To maximize profits, vehicle operators normally load their vehicles as close as possible to the legal load limit. In order to do this, the vehicle operator must be able to accurately measure the vehicle's weight while the vehicle is being loaded.
A device commonly used to weigh vehicles is the platform scale. The operator drives his vehicle onto a platform, and the weight of the vehicle and its load, known as the gross weight, is measured. The weight of the load is then determined by subtracting the weight of the unloaded vehicle, known as the tare weight, from the measured value. The weight on each axle may be determined by moving the vehicle until only the axle to be measured rests on the platform. A serious disadvantage of platform scales is their frequent unavailability when the vehicle is being loaded. Where the vehicle is not resting on the platform during loading, the operator must estimate the load placed on his vehicle. Only after the vehicle has been driven from the loading area to the weighing area does the operator learn the exact weight of his load. If the estimate is too low, the vehicle must return to the loading area, where part of the load is removed. If the estimate is too high, the vehicle must either return to the loading area or make a trip at a reduced profit.
The aforementioned disadvantage of platform scales has been eliminated somewhat by using portable scales placed under each wheel. Such scales must be carried from place to place by the vehicle, and although the scales are relatively portable, their weight and bulk reduce the load capacity of the vehicle. Further, it is often somewhat difficult and time-consuming to place these scales beneath the wheels of the vehicle.
To eliminate the aforementioned problems, on-board vehicle weighing systems have been developed. In these systems, strain gauges are generally secured to structural members of the vehicle, usually the axles. However, strain gauges placed on tandem axles to measure loading are inherently inaccurate. This is because axle bending is a function of how the tires meet the vehicle supporting surface. For a given load, axle deformation, and hence the measured load, is greater where the vehicle's weight is concentrated at the outside edges of the tire. Since the interface between the vehicle's tires and the tire supporting surface is not generally well defined, it is impossible to control the measurement accuracy of these systems.
Other on-board load measuring systems utilize load cells placed on structural members intermediate the axles and the load. However, these systems also have shortcomings. For example, they are difficult to retrofit on existing vehicles and cannot sense the extra weight added to a vehicle by dirt and other debris collected on the surfaces of the vehicle suspension beneath the load cells.
Other on-board load measuring systems measure the relative displacement of the springs or the relative displacement between the vehicle frame and axle. However, the accuracy of these devices is adversely affected by wear, friction and debris, and installation thereof generally requires relatively extensive modification of the vehicle.